1. Field of the Invention
The present invention relates generally to video signal decoders and, more specifically, to methods and systems for detecting horizontal sync pulses in video signals.
2. Related Art
Generally, video pictures or video signals are made up of video content signals, horizontal sync pulses and vertical sync pulses. Typically, a video picture includes a number of video frames. For example, according to the NTSC (National Television System Committee) format, there are 30 frames per second, and according to the PAL (Phase Alternation by Line) format, there are 25 frames per second. At the end of each frame, a vertical sync pulse is transmitted which indicates to a recipient electronic device that the frame has come to an end. The duration of the vertical sync pulse depends upon the time the electronic devices take to receive the next frame. The amplitude of the vertical sync pulse is approximately 0.3 volts, which when added to the video content signal gives a total amplitude of approximately 1.0 volt peak to peak.
Further, each video frame is made up of lines. In NTSC, there are 525 lines per frame, whereas, in PAL, there are 625 lines per frame. Each point in the line reflects the intensity of the video signal. At the end of each line, a horizontal sync pulse is transmitted which indicates to the recipient electronic device that the line has come to an end, so that the electronic device gets ready for the next line. The amplitude of the horizontal sync pulse is approximately 0.3 volts.
The screens of most monitors of electronic devices are drawn in a series of lines, left to right and top to bottom. When the monitor finishes drawing one line and reaches its right-most excursion, the beam is turned off while the monitor returns the beam to the left side of the screen. A similar process occurs when the last line on the screen is finished drawing, in which event, the beam traverses to the top left corner of the screen.
In a video picture, the beam is moved to the left of the screen and to the top of the screen in accordance with and based on the detection of the synchronization signals. In other words, when the vertical sync pulse is detected the beam moves to the top left corner of the screen to begin drawing the next frame, and when the horizontal sync pulse is detected, the beam moves to the left of side of the screen to begin drawing the next line. Accordingly, it is quite important to properly detect the sync pulses.
FIG. 1 illustrates video signal line 100, which includes horizontal front porch 110, horizontal sync pulse 120, horizontal back porch 130, color burst 132 and horizontal active pixels 150. As shown, video signal line 100 begins at the falling edge of horizontal sync pulse 120 and ends at the falling edge of the next horizontal sync pulse. Horizontal front porch 110 is the period of time between the previous horizontal active pixels (not shown) and the beginning of horizontal sync pulse 120. Horizontal sync pulse 120 is a change in voltage of the video signal, which triggers the electronic device to stop the rightward progress of drawing the beam and begin drawing on the left side of the screen. Thus, each line begins with the start of the horizontal sync pulse and ends with the start of the next horizontal sync pulse. Horizontal back porch 130 is the period of time between the end of horizontal sync pulse 120 and the beginning of horizontal active pixels 150. According to NTSC and PAL formats, horizontal back porch 130 also includes color burst 132, as a color calibration reference.
The falling and rising edges of horizontal sync pulse 120 are typically defined at 50% of the height of horizontal sync pulse 120 amplitude, which may also be referred to as the “slice level.” In other words, the falling edge of horizontal sync pulse 120 is detected when the video signal level moves below the slice level, and the rising edge of horizontal sync pulse 120 is detected when the video signal level move above the slice level. Conventionally, video decoders determine the slice level, which is used to synchronize their displays to horizontal sync pulse 120, by relying upon the amplitude of various portions of the video signal, as defined by the video standards, or by relying upon the relative amplitude of horizontal sync pulse 120 height to the peak of horizontal active pixels 150. These conventional approaches suffer from many drawbacks.
For example, because some video sources do not adhere to video standards with respect to the amplitude of the pulses, either absolutely or relative to the video content, conventional video decoders, which rely upon the absolute amplitude of the pulses or relative amplitude of the pulses, are not able to determine the slice level properly and, thus, fail to detect horizontal sync pulse 120. For instance, one conventional approach relies upon the relative amplitude for the peak, in the active or video content region, which is about 1.2v and the minimum level at zero volt to determine the slice level; however, some non-compliant video sources have the minimum level at 0.2 volts, which causes the slice level to be calculated improperly. Therefore, conventional video decoders experience problems locking to the video signals originating from such video sources, and require constant adjustment to key parameters every time a new video source is discovered that violates the standard video format in its own special way.
Accordingly, there is an intense need in the art for decoding methods and systems that can detect horizontal sync pulse 120 even if a video source is not in compliance with the standard video format, in terms of amplitude of the pluses, either absolutely or relative to the video signals.